MySQL 分组后取每组前N条数据

DROP TABLE IF EXISTS emp;
CREATE TABLE emp (
  empno decimal(4,0) NOT NULL,
  ename varchar(10) DEFAULT NULL,
  job varchar(9) DEFAULT NULL,
  mgr decimal(4,0) DEFAULT NULL,
  hiredate datetime DEFAULT NULL,
  sal decimal(7,2) DEFAULT NULL,
  comm decimal(7,2) DEFAULT NULL,
  deptno decimal(2,0) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of emp
-- ----------------------------
INSERT INTO emp VALUES (‘7369‘, ‘SMITH‘, ‘CLERK‘, ‘7902‘, ‘1980-12-17 00:00:00‘, ‘800.00‘, null, ‘20‘);
INSERT INTO emp VALUES (‘7499‘, ‘ALLEN‘, ‘SALESMAN‘, ‘7698‘, ‘1981-02-20 00:00:00‘, ‘1600.00‘, ‘300.00‘, ‘30‘);
INSERT INTO emp VALUES (‘7521‘, ‘WARD‘, ‘SALESMAN‘, ‘7698‘, ‘1981-02-22 00:00:00‘, ‘1250.00‘, ‘500.00‘, ‘30‘);
INSERT INTO emp VALUES (‘7566‘, ‘JONES‘, ‘MANAGER‘, ‘7839‘, ‘1981-04-02 00:00:00‘, ‘2975.00‘, null, ‘20‘);
INSERT INTO emp VALUES (‘7654‘, ‘MARTIN‘, ‘SALESMAN‘, ‘7698‘, ‘1981-09-28 00:00:00‘, ‘1250.00‘, ‘1400.00‘, ‘30‘);
INSERT INTO emp VALUES (‘7698‘, ‘BLAKE‘, ‘MANAGER‘, ‘7839‘, ‘1981-05-01 00:00:00‘, ‘2850.00‘, null, ‘30‘);
INSERT INTO emp VALUES (‘7782‘, ‘CLARK‘, ‘MANAGER‘, ‘7839‘, ‘1981-06-09 00:00:00‘, ‘2450.00‘, null, ‘10‘);
INSERT INTO emp VALUES (‘7788‘, ‘SCOTT‘, ‘ANALYST‘, ‘7566‘, ‘1982-12-09 00:00:00‘, ‘3000.00‘, null, ‘20‘);
INSERT INTO emp VALUES (‘7839‘, ‘KING‘, ‘PRESIDENT‘, null, ‘1981-11-17 00:00:00‘, ‘5000.00‘, null, ‘10‘);
INSERT INTO emp VALUES (‘7844‘, ‘TURNER‘, ‘SALESMAN‘, ‘7698‘, ‘1981-09-08 00:00:00‘, ‘1500.00‘, ‘0.00‘, ‘30‘);
INSERT INTO emp VALUES (‘7876‘, ‘ADAMS‘, ‘CLERK‘, ‘7788‘, ‘1983-01-12 00:00:00‘, ‘1100.00‘, null, ‘20‘);
INSERT INTO emp VALUES (‘7900‘, ‘JAMES‘, ‘CLERK‘, ‘7698‘, ‘1981-12-03 00:00:00‘, ‘950.00‘, null, ‘30‘);
INSERT INTO emp VALUES (‘7902‘, ‘FORD‘, ‘ANALYST‘, ‘7566‘, ‘1981-12-03 00:00:00‘, ‘3000.00‘, null, ‘20‘);
INSERT INTO emp VALUES (‘7934‘, ‘MILLER‘, ‘CLERK‘, ‘7782‘, ‘1982-01-23 00:00:00‘, ‘1300.00‘, null, ‘10‘);

select * from emp as a where 3> (select count(*) from emp where deptno = a.deptno and sal < a.sal  )  order by a.deptno ,a.sal